Unless you’re deliberately aiming for a lo-fi or retro lomography look, purple fringe in your photos is something you want to avoid. It’s more properly called chromatic aberration, and it’s not always purple–it can also be green and on some occasions closer to blue or even red.
Basically, it’s an issue with lenses where different wavelengths, or colors, get out of alignment and hit the sensor in slightly different places. One day it might be possible to build flat lenses with zero chromatic aberration. But for now it’s a character flaw of many of the lenses we use.